When Avril Lavigne and Nickelback lead singer Chad Kroeger first met, it was strictly for professional reasons – they were just supposed to write a couple of songs together for Lavigne's new self-titled album (out Nov. 5). What actually happened is another story altogether.

"We bonded like crazy. It was like we'd known each other forever," Lavigne, 29, tells PEOPLE exclusively. "It's a very intimate process, and you have to really open up when you're songwriting. When it was over and I had to go work with other people, it sucked. So I called him and asked him to come back to L.A. to do some more writing with me, and he was like, 'Hell yeah!' "

Soon the two had fallen for each other romantically – and written enough love ballads to fill several albums.